Recently, a vehicle incorporating a power storage device (such as a secondary battery or a capacitor) and running with driving force generated from electric power stored in the power storage device has attracted attention as an environmentally friendly vehicle. Such a vehicle includes, for example, an electric car, a hybrid car, a fuel cell car, and the like.
In these vehicles, in order to generate driving force from electric power stored in the power storage device, a motor is generally provided. A permanent magnet synchronous motor (PM motor) in which a permanent magnet is embedded in a rotor may be adopted as this motor. In such a PM motor, in order to minimize a current for driving the motor, magnetic force generated from a magnet may be set to be relatively strong. In such a case, however, during rotation of the motor at a high speed, an induced voltage (counterelectromotive force) in the motor generated by the magnet may become higher than an input voltage for an inverter driving the motor.
In order to address such a problem, what is called field-weakening control in which an induced voltage in a motor is lowered by increasing a field current in such a direction as weakening magnetic force of the magnet has generally been known. By using this field-weakening control, the motor can be driven continuously up to high rotation without uncontrollability.
Japanese Patent Laying-Open No. 2008-259270 (PTL 1) discloses a technique with which, in an electrically powered vehicle, in a case where a target value for an inverter input voltage is varied in such a situation as a low atmospheric pressure, from a point of view of protection of components, a threshold value for switching between cut-off of a gate of the inverter and zero torque control (field-weakening control) is varied in coordination with a voltage target value. According to Japanese Patent Laying-Open No. 2008-259270 (PTL 1), counterelectromotive force generated in a motor flows as regenerative electric power so that unintended regenerative braking can be prevented from occurring.